1. Field of the Invention
The present invention relates to two-axis measurement systems, and more particularly, to a high precision, Cartesian robot for a scanner, such as used for performing near-field antenna measurements with very precise planarity.
2. Description of Related Art
High performance antennas are becoming increasingly prevalent as spacecraft, aircraft, ship and ground vehicle mission requirements become more sophisticated. High performance antennas, in turn, require increasingly precise measurements of antenna performance. According to one prior art method, antenna measurement is conducted by placing the antenna at a remote location to measure the amplitude response characteristics of the antenna in its operational range. Typical operational distances for high gain antennas can range from fifty feet to three miles. This measurement technique, called far-field testing, suffers from limitations such as susceptibility to weather effects, ground reflections, and availability of adequate real estate.
Near-field testing has been developed as an alternative to far-field testing. A near field test is conducted in close proximity to the antenna, using a microwave probe to sample the field radiated near the antenna under test (AUT). A computer collects the amplitude and phase data sampled by the microwave probe, and calculates the far-field equivalent using a Fourier transform technique. Accurate near-field measurements require that all the significant antenna energy be sampled by the microwave probe. Highly directive antennas, such as reflectors and wave guide phased arrays, beam most of the energy in the forward direction normal to the antenna aperture. To test these types of antennas, a planar near-field robotic scanner is utilized to move the microwave probe along a planar pattern approximately normal to the antenna aperture. To accurately reconstruct the measured field, the probe samples the antenna energy at a plurality of points within a minimum spacing determined by the Nyquist sampling theorem. As the minimum wavelength decreases at increased antenna frequencies, ever-higher scanner planarity and/or positional accuracy are required. A near-field measurement system of this type is described in co-owned U.S. Pat. No. 5,408,318 to Slater, for WIDE RANGE STRAIGHTNESS MEASURING SYSTEM USING A POLARIZED MULTIPLEXED INTERFEROMETER AND CENTERED SHIFT MEASUREMENT OF BEAM POLARIZATION COMPONENTS, the subject matter of which is incorporated herein by reference. Further details of near-field scanner technology are provided in the book NEAR-FIELD ANTENNA MEASUREMENTS, by Dan Slater, published 1991 by Artech House, Inc., ISBN 0-89006-361-3, which is cataloged in the Library of Congress under Card. No. 91-2133, and particularly, Chapter Seven thereof; which book is incorporated herein, in its entirety, by reference.
The near-field measurement technique is also applicable to other types of reflecting bodies, emitters/receptors, or transducers, or other sources of waveforms, including all types of electromagnetic, optical, and acoustic waves, by selection of a suitable probe. The technique is thus effective in measuring the performance of antennas, lenses, and anechoic chambers. The measuring probe may act as both a transmitter and a receiver for measuring a reflected phase front from a reflecting body. The various wave sources discussed herein, including both reflecting or emitting bodies, are collectively referred to as antennas or transducers.
To test highly-directive antennas, such as reflectors and wave guide phased arrays, a planar near-field scanner is utilized. Precision positioning systems, such as Cartesian robots, are used to move the measuring probe along a planar scanning path, such as a raster pattern, approximately normal to the antenna aperture. To accurately reconstruct the measured field, the probe must sample points at some minimum spacing which is usually less than half the wavelength of the antenna signal (xcex/2). Therefore, to achieve an accurate near-field measurement, the precise position of the and its planarity with respect to the AUT is critical. For example, in a conventional prior art scanner, a planar precision of about 2.5xc3x9710xe2x88x925 meters (0.001 inches) rms is typically achieved over the primary central measurement region.
Furthermore, as signal wavelength becomes smaller, it becomes increasingly difficult to maintain the desired scanner planarity. Operation at higher radio-frequency (RF) frequencies, such as at about 650 GHz as used by astronomical and Earth limb sounding satellite instruments, requires higher planarity because of the sub-millimeter wavelengths at higher frequencies. Newer submillimeter wave antennas are larger, requiring that the scanner be accurate over a still larger area. In the past, high-precision scanners capable of measuring submillimeter-wavelength antennas have been constructed using massive granite platforms. Granite offers greater stability than metallic structures, such as aluminum or steel frames. Although granite slabs are not absolutely stable, granite tends to introduce positional errors that vary spatially relatively slowly, and are therefore more readily compensated for, compared to a much more random and spatially rapidly varying error pattern introduced by metallic structures. However, as scanner size increases, granite scanners according to the prior art become extremely heavy and prohibitively expensive. For example, one massive granite-type high-precision scanner capable of operating within a planarity of 5xc3x9710xe2x88x926 meters (0.0002 inches) rms over a 2.4 meter (8 feet) central scan area weighed in excess of about 2xc3x97104 kilograms (about 20 tons). Such a massive scanner is much more expensive to construct and install than typical Cartesian robot systems used for conventional scanners, which comprise aluminum or steel structures. However, metallic-structure based scanners are much less precise in planarity, and are less accurate positionally than massive granite-type scanners. Furthermore, position sensing and correction methods which are used with metallic-structure type scanners are in some cases incapable of achieving the desired planarity, and in other cases are undesirably costly or cumbersome.
Therefore, a high-precision planar scanner is needed, that is capable of the high precision of a massive granite-type scanner, but which uses less massive components is less expensive to construct, and which does not require a costly or cumbersome position sensing and correcting system.
The present invention provides a high precision Cartesian robot for a planar scanner capable of operating within a planarity as precise as massive granite scanners according to the prior art, while weighing approximately one-tenth as much. The high precision is achieved without the use of massive granite components, by use of an innovative design based on a relatively simple structure with well-defined and predictable errors, that are easily measured, and compensated for in acquired data. In particular, the present design utilizes a y-axis (vertical axis) structural member that is much straighter (i.e., having a precisely planar reference surface along its vertical axis) and more stable than corresponding steel or composite members in prior art scanners. The x-axis structural members need not be as precisely planar. The z-axis position of in the measuring probe is determinable to a high degree of precision based on the position of the measuring probe on the y-axis member, and a known, characteristic rigid-body tilt of the y-axis member as measured by a tilt sensor prior to the scanning process. Z-error, i.e., difference between the calculated z-position and an actual z-position of a measuring probe, is much less than previous scanners of comparable cost or weight. The innovative robot structure of the present invention minimizes unpredictable z-error, and thus provides an innovative and elegant solution to the problem of maintaining adequate planarity over the measurement area.
The Cartesian robot for a scanner according to the present invention preferably uses a combination of granite and steel components. The granite parts are much less massive than granite parts used in a conventional design, but still substantial enough to provide the stability granite is known for. In particular, the y-axis structural member comprises a highly planar vertical granite slab, which is unlike y-axis bridge structural members in prior art designs. The granite slab provides a reference surface for the y-carriage having a planarity of approximately 2.5xc3x9710xe2x88x926 meters (0.0001 inches) rms. Because of the stability, rigidity, and planarity of the granite slab, the z-position of the y-carriage can readily be determined with the required degree of precision from the vertical position of the y-carriage and the tilt of the z-reference slab, both of which are readily measured. Any z-error introduced by lateral shifting of the x-carriage, which carries the granite slab, is also readily measured and compensated. The granite slab is preferably held in place and supported by steel beam supports, by a pivoting bearing through its center. The tilt angle of the granite slab is adjusted by an adjustment screw at the base of the slab prior to the scanning process. The pivoting, center support system minimizes internal stresses in the vertical reference slab, and hence minimizes distortion of its reference surface. The Cartesian robot is thus capable of operating within a planarity of about 5xc3x9710xe2x88x926 meters (0.0002 inches) rms over a 2.4 meter (8 feet) square scan area, without using a control loop for z-axis positional control.
In a preferred embodiment, the x-axis structural member comprises a pair of substantially parallel granite bases. An x-carriage traveling on the granite bases carries the granite y-axis reference slab, and its steel supports. The configuration of the x-axis granite bases is consistent with conventional scanner robot technology. In particular, parallel rail-shaped granite bases, although less precise than the massive slabs used in prior art high precision scanners, are acceptable and preferred for use in embodiments according to the present invention. Considerable savings in weight and costs are thereby achieved.
A more complete understanding of the high precision Cartesian robot for a planar scanner will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings which will first be described briefly.